The present application relates to the examination of objects using different image acquisition modalities. It finds particular application to the use of ultrasound and x-rays in mammography examinations. It also relates to medical and other applications where information from multiple imaging modalities can be used to provide additional information about the structure and/or function of an object.
Radiographic imaging systems provide information, or images, of an object under examination or rather interior aspects of the object. For example, in radiographic imaging systems, the object is exposed to radiation, and one or more images are formed based upon the radiation absorbed by the object, or rather an amount of radiation that is able to pass through the object. Typically, highly dense objects absorb (e.g., attenuate) more radiation than less dense objects, and thus an object having a higher density, such as a bone or gland, for example, will appear differently than less dense objects, such as fatty tissue or skin, for example.
In medicine, radiographic imaging systems are commonly used to detect broken bones, masses, calcium deposits, etc. that are not visible to the naked eye. One type of radiographic image system is a mammography unit that generally comprises a radiation source, one or more compression paddles, and a detector array. The detector array is mounted on a diametrically opposing side of the breast tissue (e.g., the object under examination) from the radiation source and a compression paddle. The radiation source emits ionizing radiation (e.g., x-rays) that traverses the breast tissue while it is compressed. Radiation that traverses the breast tissue is detected by the detector array. In digital radiology, flat panel detectors detect the radiation, and reconstruction algorithms are used to create one or more two-dimensional (2D) images of the breast tissue in the latitudinal dimension (e.g., orthogonal to a center x-ray beam and/or substantially parallel to the detector array).
While two-dimensional, radiographic images are useful in mammography and other applications, these images provide little or no resolution in the longitudinal direction (e.g., parallel to the x-ray beam and/or orthogonal to the detector plane formed by the detectors). On a breast examination, for example, a two-dimensional, radiographic image cannot provide information about whether a mass is nearer the radiation source or the detector array. A potentially cancerous mass, for example, may be masked by a dense aspect of the object, such as a gland, if the mass is on top of the gland or vice versa, for example. Moreover, because objects of interest (e.g., cancerous cells) can share similar density information as objects that are not of interest, objects that are not of interest are sometimes mistakenly classified as an object of interest, resulting in a false positive.
Ultrasound imaging is one common, additional method used to confirm or reject an initial positive finding. Typically, an ultrasound probe transmits high-frequency ultrasound waves (e.g., pulses) into the object under examination. As the ultrasound waves travel through the object, they propagate differently through various tissues. The ultrasound waves that are reflected (e.g., echoes) are detected by the probe, and an ultrasound subsystem calculates the distance from the probe to the objects and/or the intensities of the echoes. An image of the target inside the breast is formed based upon the calculations.
While current cancer screening techniques have proven effective for detecting early signs of cancer in some situations, there remains room for improvement. The radiographic examination and ultrasound examination are typically done at different times and in different physical positions. For example, in breast cancer screening, the mammography exam is usually done with a woman standing upright and the breast tissue in a compressed state, while the ultrasound examination is done with the woman flat on her back and the breast stretched out (e.g., to reduce the distance the ultrasound waves have to travel into the breast, thereby improving the image quality). Therefore, it is difficult to compare the ultrasound images with the radiographic images and to detect similar details in the radiographic and the ultrasound images. Additionally, initial false positives can generate feelings of anxiety or distress that can last well after the ultrasound examination confirms that the initial positive finding was false.